A Silicon Ratchet to Produce Power from Below-Bandgap Photons

Bryan Lau, Ofer Kedem, Mohamad Kodaimati, Mark A. Ratner, Emily A. Weiss*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


This paper computationally demonstrates a new photovoltaic mechanism that generates power from incoherent, below-bandgap (THz) excitations of conduction band electrons in silicon. A periodic sawtooth potential, realized through elastic strain gradients along a 100 nm thick Si slab, biases the oscillatory motion of excited electrons, which preferentially jump and relax into the adjacent period on the right to generate a net current. The magnitude of the ratchet current increases with photon energy (20, 50, and 100 meV) and irradiance (≈MW cm−2), which control the probability of photon scattering, and peaks as a function of the well depth of the ratchet potential, and the dominant mode of energy loss (the 62 meV intervalley phonon). The internal power conversion efficiency of the ratchet has a maximum of 0.0083% at a photon energy of 100 meV, due to inefficiencies caused by isotropic scattering. This new photovoltaic mechanism uses wasted below-bandgap absorptions to enhance the directional diffusion of charge carriers and could be used to augment the efficiency of traditional photovoltaics.

Original languageEnglish (US)
Article number1701000
JournalAdvanced Energy Materials
Issue number22
StatePublished - Nov 22 2017


  • below-bandgap
  • infrared
  • photovoltaics
  • ratchet
  • silicon

ASJC Scopus subject areas

  • Renewable Energy, Sustainability and the Environment
  • Materials Science(all)


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